1. Field of the Invention
This patent specification generally describes a method and apparatus for image forming. More particularly, this patent specification describes a method and apparatus for image forming capable of correcting a misregistration of an image in color in an effective and precise manner.
2. Background Art
In general, a background color image forming apparatus according to an electrophotographic method sequentially forms a plurality of images in primary colors and superimpose them one on another so as to form a full color image. The background apparatus employs an intermediate transfer process to transfer the sequentially formed primary color images into a complete full-color image. This process typically uses an intermediate transfer member (e.g., an intermediate transfer belt) onto which the sequentially formed primary color images are superimposed one on another. This method, however, may generate an image unevenness when transfer positions of the sequentially formed primary color images are shifted relative to the intermediate transfer member, resulting in a deterioration of an image quality.
One example of the background color image forming apparatus has attempted to correct a shift of the transfer position. This example background apparatus attempts to reduce a dirt, for example, a toner on an optical detection mechanism so as to improve a detection accuracy of a reference image of each color. Based on the accurate detection of reference image, the example background apparatus performs a color registration or a density control so as to create a high quality color image. This example background apparatus is simply provided with a slidable dirt cover with a detection hole. The slidable dirt cover is disposed between the detection mechanism and the transfer medium so as to protect a sensor of the detection mechanism from contaminants such as dirt particles of toner, paper, etc.
Another example of the background color image forming apparatus has attempted to correct the above-described shift of the transfer position by forming and detecting a test pattern on the intermediate transfer belt. However, the intermediate transfer belt is typically formed in, a loop shape, having a join portion extended along in a belt width direction. Such a joint portion may be of detectable sign and become a cause of an erroneous detection of the test pattern. The example background apparatus attempts to avoid a detection of a joint sign on the intermediate transfer belt as the test pattern. Accordingly, this example background apparatus reduces an occurrence of a malfunction caused by improper detection of a flaw on the belt or an omission of patch detection so that a correction control for the color shift may be provided. The example background color image forming apparatus forms a color matching patch on the intermediate transfer belt, and obtains positional relation information of each photoconductor from a color matching patch detection signal so as to control a color matching. The color matching patch detection signal is an electric signal converted from the color matching patch by an optical sensor which is disposed at a location where the color matching patch is not formed in a belt width direction.
To attempt to eliminate noise information of the sign excluding the patch in the belt width direction, the example background apparatus is further provided with a detection mechanism, a patch detection mechanism, and an elimination mechanism. The detection mechanism detects position information of the sign (e.g., the joint sign) in the belt width direction except for the patch. The patch detection mechanism detects the position information of the patch including the sign. The elimination mechanism eliminates the position information detected by the detection mechanism from the position information detected by the patch detection mechanism.
FIG. 1 illustrates a set of parallel patches PN101 of KMCY and a set of diagonal patches PN201 of KMCY as the patches on a transfer belt 510. The KMCY indicates colors of black, magenta, cyan, and yellow. As shown in FIG. 1, a combination of the parallel and diagonal patches has often be used. Thereby, the parallel and diagonal patches PN101 and PN201 of KMCY are formed on the transfer belt 510, and are detected by a position detector 200 (e.g., the optical sensor) so that the misregistration may be corrected.
However, the background color image forming apparatus having the patch detection mechanism has increased in complexity of a configuration thereof due to a digital process needed to detect positions of the patches PN101 and PN201 after an analog-to-digital conversion of the detection signal. The digital process includes a variety of processes which need a computation by a central processing unit (CPU) so that a CPU-load is increased. The transfer position is corrected based on misregistration information relating to the patch of each color which is formed in a traveling direction of an image carrying member (referred to as a sub-scanning direction). However, the transfer position may not be accurately corrected if the transfer belt 510 involves a rotational fluctuation.